FIG. 4.

A proximal AG determines whether a distal AG is selected. (A) An AG at 15 nt determines selection of distal AGs. Splicing reactions were performed using pre-mRNAs containing duplicate AGs at 18 and 21 nt from the BPS (lane 1) or containing a third AG inserted at 15 nt from the BPS (lane 2). Lane 3 shows splicing of pre-mRNA 15/21, which serves as a marker for the lariat. (B) Other purine dinucleotides at 15 nt do not affect selection of distal AGs. Pre-mRNAs are identical to that shown in panel A (lane 2) except that GG (lane 1) or GA (lane 2) replaces the AG at 15 nt. Pre-mRNAs 15AG and 18AG are markers for the lariats (lanes 3 and 4).